JR Rivers wrote:
> Not that I object to your analysis; however...
>
> I believe that the MLT-3 used in FDDI-TPPMD and 100BASE-TX can also be
> considered multi-level. If you agree, then it can serve as an example of a
> multilevel coding scheme signalled over copper with a BER of 10e-12. I
> also believe that the 8B6T signalling scheme in 100BASE-T4 is multi-level;
> however, I don't know what it's BER ended up being.
>
> JR
>
Hello JR,
100BASE-TX is surely included. However, its target BER
seems to be more 2.5*10(-10) than 10^(-12).
The 100BASE-TX standard does not include explicitly
the BER objective under the "PCS objectives", as it usually
does for the other Ethernet Copper standards.
The target BER seems to be 2.5*10^(-10), according to
ANSI X3.166-1990, under paragraph 8, page 20, "Media
signal interfce:
"The parameters speciified in this clause are based
on a requirement that the bit error rate contributed
by the repetition though an FDDI attachment shall
not exceed a bit error rate of 2.5 x 10^(-10) under all
conditons of clause 8, including the minimum Active
Input Interface power level. In addition, the FDDI
attachment shall not exceed a bit error rate of
10^(-12) when the Active Input Interface power
level is 2 dB or more above the minimum level."
This BER interpretation of 2.5 * 10^(-10) for
100BASE-TX is also consistent with what I found
in Howard W. Johnson's book "Fast Ethernet, down
of a new network". Howard writes on page 140, under
Error Robustness:
"The residual probablity of an X receiver [100BASE-X]
failing to detect any combination of errors is extremely
small. Assuming a uniform line error of one part in
2.5*10^10, the mean time between undetected packet
errors is 31 million billion years."
The interpretation of "uniform line error" as the usual
BER for 100BASE-TX receives further confirmation when
Howard says in page 121, on Error Robustness referring
to 100BASE-T4:
"The residual probability of a T4 receiver failing to
detect any combination of errors is extremely small.
Assuming a uniform line error rate of one part in
10^8, the mean time between undetected packet errors
would be 307 million billion years."
A BER of 10^8 is exactly what is stated in the Ethernet
standards under the "Objectives" section in the PCS
for the 100BASE-T4 Standard.
Jaime
Jaime E. Kardontchik
Micro Linear
San Jose, CA 95131
>
> At 04:36 PM 2/27/00 -0800, Jaime Kardontchik wrote:
> >
> >Hello 10G'ers,
> >
> >Edward Chang was right on target.
> >He wrote on Feb 23:
> >
> >> In the past, the multiple voltage-level coding
> >> was adopted by two LAN standards, ATM and
> >> Ethernet. Both of them were twisted-pair
> >> applications, and the BER were 10^(-10).
> >> I proposed BER of 10^(-12) in both working
> >> groups to be consistent with the LAN optical
> >> links' BER; however, for some reason, they
> >> remain 10^(-10), officially in both standards.
> >
> >What was the reason ?
> >
> >Multilevel voltage coding was not the reason.
> >On the contrary, it was a remedy. The real reason
> >was the strong ISI at the maximum link lengths
> >that these Standards wanted to support.
> >Multi-level voltage coding was adopted to
> >lower the baud rate or the frequency content
> >of the signal in order to make the ISI smaller
> >and get a better BER.
> >
> >However, the remaining ISI in these Copper links
> >remained high enough that the eye at the
> >input of the receiver remained still
> >completely closed. It is this closed eye that
> >limited the achievable BER in the Copper media
> >to 10^(-10). Post-equalizers, no matter how
> >sophisticated they were, are not able to
> >completely reverse the effects of eye closening
> >on the achievable BER.
> >
> >---> Conclusion ? In order to get a BER of
> > 10^(-12) you have to have a clear
> > open eye at the input of the receiver.
> > The maximum supported link length
> > is set accordingly to meet this basic
> > condition: open eye.
> >
> >Now we can go to the basic two PAM-5 proposals:
> >
> >1) PAM-5 serial at 5 Gbaud
> >
> >Using 1300 nm lasers lasers at 5 Gbaud the
> >optical eye is completely closed already at
> >~170 meters due to heavy ISI (installed MMF,
> >500 MHz*km bandwidth).
> >
> >Oscar claimed a support of 500 meters of
> >installed MMF and pointed out that
> >DFEs (equalizers) have been successfuly used
> >in 100 and 1000 Mbps Copper networks.
> >However, the specs and experience of the 100 Mbps
> >and 1000 Mbps links over Copper - where the eye
> >at the input of the receiver is completely
> >closed at the target link length due to ISI -
> >put the achievable BERs around 10^(-10) only.
> >Even using sophisticated equalizers.
> >
> >What is the experimental support that a BER
> >of 10^(-12) could be achieved when a strong ISI
> >closes completely the eye at the input of the
> >receiver ? I think that none and that the
> >experience points to the contrary. Why then
> >could an equalizer running at 5 GHz achieve
> >here what sophisticated equalizers running
> >40 times slower (125 MHz) were not able to achieve
> >in the Copper Ethernets ? Parallel processing
> >could enable perhaps to meet the timing
> >constraints of the design by running multiple
> >equalizers at a lower clock, but will not
> >eliminate the basic limitation on achievable
> >BERs once the eye at the input of the receiver
> >is already completely closed.
> >
> >Let us see now the case PAM-5 at 1.25 Gbaud:
> >
> >2) PAM-5 4-WDM at 1.25 Gbaud
> >
> >Let us compare this approach to two other well
> >known on-off approaches: 1 GbE and 4-WDM at
> >3.125 Gbaud using the 8b/10b coding. Let us
> >assume again 1300 nm lasers and installed
> >500 MHz*km MMF.
> >
> >The figures below (I hope will not get distorted
> >during transmission) show the power levels of
> >the three systems. For 1 GbE I assumed no ISI.
> >For PAM-5 I assumed 400 meters link length
> >(there is no ISI up to this distance, ISI= 0 dB).
> >For 4-WDM at 3.125 Gbaud I assumed 300 meters link
> >length and about 3 dB optical loss due to ISI.
> >
> >| |
> >| |
> >| | 1 GbE, 1.25 Gbaud, no ISI
> >| |
> >| |
> >
> >
> >| | | | |
> >| | | | |
> >| | | | | PAM-5 4-WDM at 1.25 Gbaud
> >| | | | | no ISI.
> >| | | | |
> >
> >|**** ****|
> >|**** ****|
> >|**** ****| 8b/10b 4-WDM at 3.125 Gbaud
> >|**** ****| 3 dB ISI loss
> >|**** ****|
> >
> >( the asterisks denote closening of the eye due
> >to ISI)
> >
> >2a) 1 GbE
> >
> >By definition, the optical power difference
> >between the '0' and '1' levels in the 1 GbE
> >case is 1:
> >
> > optical signal power = 10*log(1) = 0 dB
> >
> > optical SNR = 0 dB (reference)
> >
> >And we get a BER of 10^(-12).
> >
> >2b) PAM-5 4-WDM at 1.25 Gbaud
> >
> >In the PAM-5 case, notice that using the
> >"open fiber control" method that I described
> >in a previous email, we get the same launched
> >power per channel as in the 1 GbE Ethernet.
> >However, in PAM-5 the power difference
> >between levels is 0.25 (this is the well
> >known 6 dB optical power penalty loss of
> >the PAM-5 modulation). Let us now add
> >the 6 dB electrical coding gain provided
> >by the Viterbi decoding: the effective distance
> >between levels doubles. Summarizing, in PAM-5
> >4-WDM at 1.25 Gbaud, the effective optical
> >signal power difference between levels is:
> >
> > effec optical signal power diff =
> > 10*log(2*0.25) = - 3 dB
> >
> >where the factor 2 inside the log comes
> >from the coding gain.
> >
> >The noise power at the input of the
> >receiver is the same as in 1 GbE because
> >we use the same baud rate. Hence,
> >
> > effec optical SNR = - 3 dB
> >
> >This is not bad compared to the 0 dB of 1 GbE.
> >Furthermore, notice that we could even bring
> >back the SNR for PAM5 to 0 dB (the 1 GbE
> >reference) if a new laser safety proposal
> >to move the maximum safe power from -4 dBm
> >to -1 dBm is accepted. See P. Kolesar et al
> >presentation in the next March meeting.
> >
> >2c) 8b/10b 4-WDM at 3.125 Gbaud
> >
> >In this case the eye is half closed due
> >to ISI. Hence the optical power difference
> >is:
> >
> > optical signal power = 10*log(0.5)
> > = - 3 dB
> >
> >and the electrical noise power at the input
> >of the receiver is larger, since the receiver
> >needs more bandwidth:
> >
> > elec noise power = 10*log[(3.125/1.25)^2]
> > = 8 dB
> >
> >Hence, the optical SNR is
> >
> > optical SNR = - 3 - 8/2 = -7 dB
> >
> >This case is clearly worse than the 1 GbE
> >case in terms of optical SNR. And the
> >Task Force considers that this system
> >can achieve the needed 10^(-12) BER.
> >
> >Summarizing:
> >
> > PAM-5 4-WDM at 1.25 Gbaud has no
> > ISI, a clear and wide open eye at
> > the input of the receiver, an optical SNR
> > only slightly less than the SNR of present
> > 1 GbE transceivers and better than
> > the proposed 8b/10b 4-WDM at 3.125G
> > transceivers.
> >
> > Hence, it is a good choice if one
> > wants to reach the required BER of
> > 10^(-12).
> >
> >Note: the above analysis, based on simple
> > static SNRs, is not a substitute for a
> >much more careful analysis regarding
> >the viabibility of any PAM-5 approach.
> >However, a simple back-of-the-envelope
> >analysis is very useful to make a quick
> >comparison between different PAM-5 proposals
> >in order to find out which PAM-5 architecture,
> >
> > a) one working under strong ISI conditions
> > that close the optical eye completely; or
> >
> > b) another using a lower baud rate
> > and working with minimal ISI so that
> > the optical eye at the input of the
> > receiver is widely open,
> >
> >which one has more chance to meet the required
> >BER of 10^(-12), and discard accordingly the
> >one that does not have a chance.
> >
> >Jaime
> >
> >Jaime E. Kardontchik
> >Micro Linear
> >San Jose, CA 95131
> >email: kardontchik.jaime@xxxxxxxxxxx
> >
> >
> >